In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit dynamic random access memory (DRAM, processing feature size 0.25 μm or less), the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 M and 1 G or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source is required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724).
Several problems associated with the presence of water on resist were pointed out. For example, profile changes occur because the acid once generated from a photoacid generator and the amine compound added to the resist as a quencher can be dissolved in water. The pattern collapses due to swelling. It was then proposed to provide a protective coating between the resist and water (see the 2nd Immersion Workshop, Jul. 11, 2003, Resist and Cover Material Investigation for Immersion Lithography).
In the lithography history, the protective coating on the resist layer was studied as an antireflective coating. For example, the antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCOR process involves forming a transparent antireflective coating on top of a resist film and stripping it after exposure. Despite its simplicity, the process can form a micropattern at a high degree of definition, precision and alignment. When the antireflective coating is made of perfluoroalkyl compounds (e.g., perfluoroalkyl polyethers or perfluoroalkyl amines) having a low refractive index, the light reflection at the resist/antireflective coating interface is minimized so that the dimensional precision is improved. In addition to these materials, the fluorinated materials proposed thus far include amorphous polymers such as perfluoro(2,2-dimethyl-1,3-dioxol)-tetrafluoroethylene copolymers and cyclic polymers of perfluoro(allyl vinyl ether) and perfluorobutenyl vinyl ether as disclosed in JP-A 5-74700.
Because of their low compatibility with organic substances, the foregoing perfluoroalkyl compounds must be diluted with fluorocarbon solvents such as Freon for controlling a coating thickness. As is well known in the art, the use of fluorocarbons now raises an issue from the standpoint of environmental protection. The perfluoroalkyl compounds are awkward to form uniform films, and are not regarded satisfactory as antireflective films. Additionally, the antireflective films must be stripped with fluorocarbon solvents prior to the development of photoresist. These factors lead to many practical disadvantages including a need to add an antireflective film-stripping unit to the existing system and the increased cost of fluorocarbon solvents.
If the antireflective film is to be stripped without adding an extra unit to the existing system, it is most desirable to carry out stripping in the development unit. The solutions used in the photoresist development unit are an alkaline aqueous solution as the developer and deionized water as the rinse. It would be desirable to have an antireflective coating,material which can be readily stripped with such solutions.
For this reason, there were proposed a number of water-soluble antireflective coating materia processes using the same. See, for example, JP-A 6-273926, JP-A 6-289620, and JP-A 7-160002.
The water-soluble protective coatings, however, cannot be used in the immersion lithography because they are dissolved away in water during light exposure. On the other hand, water-insoluble fluoro-polymers pose a need for special fluorocarbon strippers and an exclusive stripping cup for fluorocarbon solvents. It was thus desired to have a resist protective coating which is water insoluble, but can be readily stripped.
The ideal refractive index of an antireflective coating on resist is a square root of the refractive index of air multiplied by the refractive index of the resist film. Since ArF resist films based on methacrylate and cycloolefin polymers have a refractive index of approximately 1.72 at 193 nm, for lithography in air, the optimum refractive index of the overcoat film is a square root of 1.72, which is calculated to be 1.31. For immersion lithography wherein the immersion medium is water, for example, the optimum is a square root of the refractive index 1.44 of water multiplied by the refractive index 1.72 of the resist film, which is calculated to be 1.57.
Fluoro-polymers, which are reported in the 2nd Immersion Workshop, Jul. 11, 2003, Resist and Cover Material Investigation for Immersion Lithography, have a low refractive index of 1.38 which is far off the optimum value.
The inventors performed a simulation of immersion lithography at wavelength 193 nm, with the results shown in FIGS. 1 to 9. Assume a layer structure including, in sequence, a Si substrate, an antireflective coating (BARC) having a refractive index (n) 1.5, an extinction coefficient (k) 0.4, and a thickness 85 nm, a resist layer having a refractive index 1.72, and a resist protective coating (or top anti-reflective coating, TARC). When the refractive index and thickness of TARC and the thickness of the resist layer were varied, the reflectivity from TARC to water was computed. The reflectivity varies periodically as the thickness of TARC and the thickness of resist vary. The thickness of TARC (pointed by the arrow) when the reflectivity of resist becomes minimum is the optimum TARC thickness. The target value of reflectivity is set at 2% or less (reflectivity≦0.02). Where the refractive index of TARC (=1.3, 1.4) is lower than that of water, the reflectivity exceeds 4%. A reflectivity of 2% or less is obtained when the refractive index of TARC is 1.55, 1.60 and 1.65. It is thus seen that a refractive index of approximately 1.57 is the optimum.